1. Field of the Invention
This invention relates to a process for solvent dewaxing waxy hydrocarbon oils using a dewaxing aid. More particularly, this invention relates to a solvent dewaxing process for waxy hydrocarbon oils using a polyvinylpyrrolidone dewaxing aid. Still more particularly this invention relates to a ketone solvent dewaxing process for bright stocks employing a polyvinylpyrrolidone dewaxing aid having a number average molecular weight ranging from about 40,000 to 400,000.
2. Description of the Prior Art
Waxes in wax-containing hydrocarbon oils are removed therefrom by chilling the oil to precipitate out the wax and then separating the solid wax particles from the dewaxed oil by filtration or centrifugation. Industrial dewaxing processes include press dewaxing processes wherein the wax-containing oil, in the absence of solvent, is chilled to crystallize out the wax particles which are then pressed out by a filter. In general, only light hydrocarbon oil fractions (paraffinic fractions) obtained by vacuum distillation are treated by press dewaxing processes due to viscosity limitations. More widely used are solvent dewaxing processes wherein a waxy oil is mixed with a solvent and then chilled to precipitate the wax as tiny particles or crystals thereby forming a slurry comprising wax particles and a solution of dewaxed oil containing dewaxing solvent. The slurry is then fed to a wax filter wherein the wax is removed from the dewaxed oil and dewaxing solvent. Solvent dewaxing processes are used for heavier oil fractions such as lubricating oil fractions and bright stocks. Typical dewaxing solvents include ketones such as mixtures of acetone and MEK or MEK and MIBK as well as mixtures of ketones and aromatic hydrocarbons such as MEK/toluene and acetone/benzene.
One of the factors tending to limit the capacity of a solvent dewaxing plant is the rate of wax filtration from the dewaxed oil, which in turn is strongly influenced by the crystal structure of the precipitated wax. Although the crystal structure of the precipitated wax is influenced by various operating conditions in the dewaxing process, for any given feed it is most strongly influenced by the chilling condition. The size and crystal structure of the precipitated wax, occlusion of oil in the wax crystals and of the condition of the oil left in the crystal are extremely varied and depend on the wax composition and precipitation condition. These conditions also affect the filtration rate of the dewaxed oil from the wax and the yield of dewaxed oil. In some cases, most notably when the waxy oil is a bright stock, the wax crystals are of an extremely fine size and not all are separated by filtration, but some leave the filter with the dewaxed oil component which creates an objectionable haze in the oil.
One way of increasing the filtration rate and minimize haze formation is to add a dewaxing aid to the wax-containing oil. Well known in the art are dewaxing aids such as .alpha.-olefin copolymers, mixtures of materials such as a mixture of (a) an ethylene-vinyl acetate copolymer and (b) an unsaturated ester of an aliphatic alcohol having from 2 to 20 carbon atoms with acrylic or methacrylic acid, as well as polymeric dewaxing aids comprising condensation products of chlorinated paraffins and naphthalenes alone or mixed with acrylic ester polymers. However, in the case of heavy stocks, these dewaxing aids are not too efficient, therefore necessitating relatively high concentrations of the dewaxing aid in the oil. This is especially true when a heavy oil raffinate or a bright stock is solvent dewaxed. When these oils are solvent dewaxed, a portion of the wax is precipitated as crystals so fine that they pass through filter cloths thereby creating a haze in the dewaxed oil which greatly reduces the commercial value of same. Also, because of the presence of so many fine particles of wax, the filter rate of the dewaxed oil tends to be lower than that obtained with lighter lube oil stocks. Therefore, there is a need for efficient dewaxing aids for use with heavy stocks.